Historic buildings constitute a considerable share of the European building stock. About 35% of the EU's buildings are over 50 years old, with 26.4% dating from before 1945 . Improving the energy performance of historic buildings (i.e. buildings built before 1945) improves the internal comfort conditions while reducing the energy demand and therefore reduces the risk of fuel poverty. Providing users with current comfort standards is a crucial requirement to ensure the continued use of historic buildings over time and their conservation and endurance.
In an attempt to lower the energy consumption from buildings, the Directive 2018/844 has reinforced the target of ‘nearly zero-energy’ for new buildings and existing buildings subject to major renovations. However, a prejudice widespread across many countries is that historical buildings, and particularly those under monument protection, should not be equipped with new energy efficient technologies, and should thus be exempted.
Any renovation needs to follow three main steps in order to ensure the achievement of sustainability and energy efficiency targets:
- To ensure the maximum material preservation, it is preferable to intervene in historic buildings where material degradation adds up to large parts of the historic building,
- To minimise landscape alteration, it is desirable to intervene on shelters and service volumes annexed to or in the vicinity of historic buildings (rather than on historic buildings),
- In the urban landscape it is preferable to intervene on buildings already physically compromised or on adjacent recent buildings .
It is possible to identify some lines of action for energy related interventions in historic buildings. These can range from maximum preservation (rather conservative) to a rather dynamic approach that keeps some parts of the historic building that are considered to embrace outstanding aesthetical values .
With regards to insulation, two strategies could be followed:
- When interior features are to be preserved or when external damage requires reconstruction, external insulation is a viable option. Several Case studies preserving the interior rooms features and façade rebuild have already been presented by BuildUp.
- Historic listed buildings often have facades worthy of preservation. In this case the approach to energy retrofit will be of providing internal insulation. This is also the case when technical limitations do not allow for the installation of external insulation, such as the presence of close neighbouring buildings, insufficient space, etc. However, when internal insulation is applied, the hygrothermal characteristics may change and moisture management is therefore a major task for the development of interior insulation systems .
Two common approaches to moisture management are:
- using vapour-impermeable systems, and
- using vapour-permeable, capillary-active systems.
Using vapour-impermeable interior insulation systems prevents the water vapour flux in the wall with vapour-retardant foils, dense interior plaster, etc. The positive aspect of this type of insulation is that there is no condensation inside the wall. However, it is still possible to have migrating moisture that cannot dry out towards the inside, for example, when driving rain penetrates the wall. In this case, the use of vapour-retardant systems, where the vapour conductivity changes depending on the surrounding moisture level, are employed. For this reason, and in consideration of historic buildings, this article will propose what could be considered a more appropriate approach, which is the following.
When using vapour-permeable, capillary-active, interior insulation systems vapour is allowed to diffuse into walls; they buffer the resulting moisture and remove it from the condensation zone back towards the room. The hygroscopic storage capacity of vapour-permeable, capillary-active interior insulation systems allows for humidity in indoor air to be buffered. Crucial to the functioning and performance of the interior insulation is the interaction between (a) moisture buffering, (b) vapour and (c) liquid water transport.
An assessment of interior insulation types therefore requires exact knowledge of these variables and more sophisticated measurements than usual. A recently published paper  has employed simulation methods to explore the hygrothermal risks of internal insulation in addition to a life-cycle assessment to investigate the environmental impact of the intervention. Importantly, when applying internal insulation, regulations require both moisture protection and fire protection. In addition, detailed planning is required to analyse the state of the construction, to prepare the surfaces, to protect against the driving rain on the exterior wall, and to design for airtightness and cold bridges .
A good insulation solution will provide a proper moisture buffer to regulate the room climate, a large drying potential to reduce freezing damage, and resistance against mould. Some vapour-permeable capillary-active interior insulation systems are described.
- Mineral foam interior insulation system, made from a material that is highly vapour-permeable, with good moisture storage and moderate moisture transport. The insulation board is easily workable. The system is non-combustible and offers a high degree of fire protection. The measured thermal conductivity is λ = 0.042 W/mK.
- Perlite interior insulation system is a capillary-active interior insulation based on natural expanded Perlite, with good moisture transport at high levels of humidity and moderate moisture storage. The system is not combustible and offers a high degree of fire protection. The measured thermal conductivity is λ = 0.045 W/mK. A case study for this system can be found here.
- PUR interior insulation system offers an optimal energy-saving option for interior insulation since this product has a low thermal conductivity of λ = 0.031 W/mK. Also, the board’s vapour-diffusion resistance is several times higher than other capillary-active systems, so there is less risk of having condensation formed inside the envelope walls. However, as the board is made from polyurethane, its resistance and reaction to fire needs to be carefully considered to avoid extra risks.
- Calcium silicate ‘climate board’. Calcium silicate climate board in combination with a moisture-regulating smooth lime plaster finish is an interior insulation system that has been known for a long time. However, with its thermal conductivity of about λ = 0.06 W/mK, greater thickness than other such systems are required to obtain comparable insulation values. These climate boards are notable for effective moisture transport. The system is not combustible and offers a high degree of fire protection. With the upgraded version of Calcium silicate ‘Xtra climate board’ the thermal conductivity is halved to λ = 0. 0343W/mK.
- Loam cork kieselguhr interior insulation is an ecologically friendly alternative to interior insulation systems. This thermally insulating clay (mixture of clay, expanded cork, kieselguhr and wood wool) has a higher thermal conductivity of λ = 0.075 W/mK and is therefore not as effective at saving energy as the other insulation materials examined here. Owing to its lower insulating effect and its higher resistance to vapour diffusion, the risk of condensation inside the wall is reduced .
Energy efficient retrofit is useful for structural protection as well as for comfort reasons, comfort for users and ‘comfort’ for heritage collections. The Seventh Framework Programme project 3ENCULT explored the possibilities of alternatives for obtaining energy efficiency saving measures in historic building renovation. Importantly, this completed project has pioneered this path by providing an approach and a process that makes it possible to identify and integrate values of culture and energy in the conservation of the built heritage. In this way, the project 3ENCULT bridges the gap between the conservation of historic buildings and climate protection, which should not be considered as an antagonism at all; historic buildings will only survive if they are maintained as living spaces.
The ATLAS Project aims at paving the way for sustainable development of historic structures. This will include capitalizing and optimizing existing best practice solutions for building refurbishment and regional development. Stakeholders of the whole value chain and decision-makers are included in a network to ensure sustainability from social, ecological and cultural points of view. Here, case studies show that a reduction of a factor of 4 (i.e. reducing the energy demand by 75%) and beyond is possible in historic buildings while also preserving their heritage value. This project fosters the exchange of best practice experiences and high level knowledge from all Alpine regions on energy retrofit and sustainable regional development in order to remove uncertainties, quantify co-benefits and socio-economic value, and provide the ground for the integration of historic buildings and sites in low-carbon policies and regional development strategies.
The SHC Task 59 is a project aiming to develop a solid knowledge base on how to save energy in the renovation of historic and protected buildings in a cost-efficient way. This is pursued by identifying the energy saving potential for protected and historic buildings according to typologies, and also by identifying procedures where experts can work together with integrated design to maintain both the heritage value of the building and at the same time improve the energy efficiency.
Because insulation alone may not be enough, especially when the intrinsic nature of buildings limits insulation interventions, retrofit measures may integrate appropriate forms of renewable energy generation that follows the same guiding principles of interventions, as mentioned in the first part of this overview. One project that underpins this dialogue is the project BIPV meets history. This project is looking at eliminating barriers to the renovation of historic buildings, in this case with the inclusion of building integrated photovoltaics, by means of analysing the legislative, normative and procedural context. This project will meet the requirements of local, national and European policies while respecting the heritage and landscape values of the territory, among others.
In conclusion, the growing sector of insulating materials is progressively called to provide solutions aimed at preserving both energy in buildings as well as preserving the tangible heritage that buildings, historic places, and monuments embed as an actual source of memory, while at the same time changing its state. Thanks to the advance in customisation of technological solutions, we account today for new opportunities to retrofit heritage-protected buildings, while preserving their cultural and essential values.
 Troi A. Historic buildings and city centres, potential impact of conservation compatible energy refurbishment on climate protection and living conditions. In: Proceedings conference on energy management in cultural heritage, Dubrovnik; 2011. p. 6e8.
 Building Integrated Photovoltaic in Historic Buildings in the Solar Update newsletter, available at https://www.iea-shc.org/Data/Sites/1/publications/2019-12-Solar-Update-Newsletter.pdf
 Troi A, Bastian Z. Energy efficiency solutions for historic buildings. A handbook. Basel: Birkhauser Verlag; 2015. ISBN: 978-3-03821-646-9.
 Bottino-Leone D, Larcher M, Herrera-Avellanosa D, Haas F, Troi A. Evaluation of natural-based internal insulation systems in historic buildings through a holistic approach, Energy, 181 (2019), 521-531.